Light emitting device and driving method of the same

Background of the invention

1. Field of the invention

The present invention relates to a light emitting device in which a light emitting element and a unit for supplying current to the light emitting element are provided in each of a plurality of pixels.

2. Description of the related art

A light emitting element is highly visible since it emits light by itself A light emitting device formed by using the light emitting element does not require a backlight which is necessary in a liquid crystal display device (LCD) and it has no limit in its viewing angle. Therefore, the light emitting device formed by using the light emitting element is drawing attention as a display device which can substitute for a CRT and an LCD. In recent years, it is mounted in such electronic devices as a portable phone and a digital still camera, and its practical application is widely growing.

The light emitting device can be divided into two types: a passive matrix type and an active matrix type. The active matrix light emitting device which is becoming a mainstream is suitable for a large panel and high precision since current supply to a light emitting element can be maintained to some extent after inputting a video signal. A specific pixel configuration of the active matrix light emitting device varies according to manufacturers and each manufacturer exercises its ingenuity, however, at least the light emitting element, a transistor for controlling an input of a video signal to the pixel, and a transistor for supplying a current to the light emitting element are provided in each pixel.

Summary of the invention

These and other objects, features and advantages of the present invention will become more apparent upon reading of the following detailed description along with the accompanied drawings.

In the case where an off-current of the transistor for controlling an input of a video signal to the pixel is large, a gate-source voltage Vgs (hereinafter referred to as a gate voltage) of the transistor for controlling the amount of current to be supplied to the light emitting element tends to vary. In order to prevent the variation of the gate voltage Vgs, a capacitor having a larger capacitance may be provided between the gate and source of the transistor or the off-current of the transistor for controlling an input of a video signal to the pixel may be suppressed low. However, when the capacitor occupies a larger area, current may leak between the electrodes due to dust and the like, which leads to decrease the yield. Further, it takes cost and time to optimize the process of transistor so as to suppress the off-current of the transistor for controlling the input of the video signal to the pixel and also to increase the on-current for charging a large capacitance. It is also a problem that the gate voltage Vgs of the transistor for controlling the current supply to the light emitting element is apt to change easily in accordance with the switching of the other transistors, change in the potential of the signal line and scan line and the like due to the parasitic capacitance of the gate.

In view of the aforementioned problems, the invention provides a light emitting device in which the capacitor occupies a small area and the variation in luminance of the light emitting element caused by the variation in the gate voltage Vgs of the transistor for controlling the current supply to the light emitting element can be suppressed while using the transistors formed in the existing process.

According to the invention, not only a transistor (driving transistor) which determines a current value to be supplied to the light emitting element but a transistor (current controlling transistor) which operates as a switching element are also connected to a driving transistor in series. The driving transistor is turned ON by receiving a fixed potential to its gate at least in a period for displaying an image so that current can flow constantly. Further, the current controlling transistor operates in a linear region and its gate potential is controlled by the video signal inputted to the pixel.

When the current controlling transistor operates in a linear region, the source-drain voltage (drain voltage) Vds thereof becomes extremely small relatively to a voltage Vel applied to the light emitting element. Thus, a slight change in a gate voltage Vgs does not easily affect the current supplied to the light emitting element. The gate potential of the driving transistor is fixed without being controlled by a video signal. Therefore, the current supplied to the light emitting element does not change easily even without increasing the capacitance of the capacitor provided between the gate and source of the current controlling transistor, or suppressing the off-current of the transistor for controlling an input of a video signal to the pixel low. The current supplied to the light emitting element is not affected by the parasitic capacitance of the gate of the current controlling transistor. The current controlling transistor only operates to supply or not to supply a current to the light emitting element. The current value to be supplied to the light emitting element is determined by the driving transistor. Therefore, causes of the variation are decreased and the image quality can considerably be enhanced. Moreover, the process does not have to be optimized for suppressing the off-current of the transistor for controlling an input of a video signal to the pixel, therefore, fabrication process of the transistors can be simplified, which contributes to the reduction in cost and improvement in yield.

The driving transistor operates desirably in a saturation region in the invention, however, it may operate in a linear region as well. The drain current is apt to be affected by the slight change in the gate voltage Vgs in a saturation region more easily than in a linear region. However, the gate potential of the driving transistor is fixed in the invention, therefore, the gate voltage Vgs does not change even when the driving transistor operates in a saturation region. When the driving transistor operates in a saturation region, the drain current is not changed by the drain voltage Vds, but determined only by Vgs. Therefore, value of the drain current is maintained relatively constant even when Vds becomes small instead of Vel becoming large in accordance with the degradation of the light emitting element. Therefore, the reduction in luminance of the light emitting element and the appearance of the luminance variation due to the degradation of the electroluminescent material can be suppressed.

The channel length L of the driving transistor may be longer than the channel width W thereof, and L of the current controlling transistor may be equal to or shorter than W thereof. More desirably, L of the driving transistor may be 5 or more times as long as W. By employing the aforementioned structure, the variation in luminance of the light emitting element between the pixels due to the difference in characteristics of the driving transistor can be further suppressed. It is assumed that the channel length and the channel width of the driving transistor are L1 and W1 respectively, and the channel length and the channel width of the current controlling transistor are L2 and W2 respectively. Then, when L1/W1:L2/W2=X:1 is satisfied, X is desirably in the range of 5 to 6000. For example, there are cases that L1/W1=500 .mu.m/3 .mu.m, and L2/W2=3 .mu.m/100 .mu.m.

The light emitting element in this specification includes an element of which luminance is controlled by current or voltage, such as an OLED (Organic Light Emitting Diode), an MIM electron source element (electron emitting element) used to a FED (Field Emission Display) and the like.

The light emitting device includes a panel in which a light emitting element is sealed, and a module in which an IC and the like having a controller are mounted on the panel. The invention relates to an element substrate corresponding to one mode that is before completing the light emitting element in the process for fabricating the light emitting device. Each of the plurality of pixels on the element substrate is provided with a unit for supplying current to the light emitting element.

The element substrate may be in a various modes such as the one that only pixel electrode of the light emitting element is formed, or the one that after forming a conductive layer as the pixel electrode and before patterning to form the pixel electrode.

The OLED as a light emitting element includes a layer containing an electroluminescent material in which luminescence (electroluminescence) is generated by applying an electric field (hereinafter, referred to as an electroluminescent layer), an anode layer and a cathode layer. The electroluminescent layer is formed by a single or a plurality of layers and provided between the anode and the cathode. At least one of the aforementioned layers may include a non-organic compound. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) are included in the luminescence in the electroluminescent layer.

A thin film transistor formed by using polycrystalline silicon and amorphous silicon can be used as a transistor used in a light emitting device of the invention, however, it is not exclusively limited to the thin film transistor. It may be a transistor formed by using single crystalline silicon or by using SOI. Moreover, it may be a transistor formed by using an organic semiconductor or a transistor formed by using carbon nanotube. A transistor provided in a pixel of the light emitting device of the invention may have a single gate structure, a double-gate structure, or a multi-gate structure having a plurality of gate electrodes.

According to the invention, the current supplied to the light emitting element does not change easily even without increasing the capacitance of the capacitor provided between the gate and source of the current controlling transistor, or suppressing the off-current of the transistor for controlling an input of a video signal to the pixel low. The current supplied to the light emitting element is not affected by the parasitic capacitance of the gate of the current controlling transistor. The current controlling transistor only operates to supply or not to supply a current to the light emitting element. The current value to be supplied to the light emitting element is determined by the driving transistor. Therefore, causes of the variation are decreased and the image quality can considerably be enhanced. Moreover, the process does not have to be optimized for suppressing the off-current of the transistor for controlling an input of a video signal to the pixel, therefore, fabrication process of the transistors can be simplified, which contributes to the reduction in cost and improvement in yield.

The driving transistor operates desirably in a saturation region in the invention, however, it may operate in a linear region as well. The drain current is apt to be affected by the slight change in the gate voltage Vgs in a saturation region more easily than in a linear region. However, the gate potential of the driving transistor is fixed in the invention, therefore, the gate voltage Vgs does not change easily even when the driving transistor operates in a saturation region. When the driving transistor operates in a saturation region, the drain current is not changed by the drain voltage Vds, but determined only by Vgs. Therefore, value of the drain current is maintained relatively constant. Therefore, the reduction in luminance of the light emitting element and the appearance of the luminance variation due to the degradation of the electroluminescent material can be suppressed.

Brief description of the drawings

FIG. 1 is a circuit diagram of a pixel of the light emitting device of the invention.

FIGS. 2A to 2D are diagrams showing driving methods of the light emitting device shown in FIG. 1.

FIGS. 3A and 3B are diagrams showing driving methods of the light emitting device shown in FIG. 1.

FIG. 4 is a diagram showing a timing of a write period and a store period of the light emitting device of the invention.

FIG. 5 is a diagram showing driving methods of an active matrix light emitting device.

FIG. 6 is a list of driving methods classified by voltage or current of the video signals.

FIGS. 7A to 7E are circuit diagrams of a pixel of the light emitting device of the invention.

FIGS. 8A and 8B are circuit diagrams of a pixel of the light emitting device of the invention.

FIGS. 9A and 9B are circuit diagrams of a pixel of the light emitting device of the invention.

FIG. 10 is a top plan view of a pixel of the light emitting device shown in FIG. 1.

FIG. 11 a top plan view of a pixel of the light emitting device shown in FIG. 1.

FIGS. 12A and 12B show cross sectional structures of a pixel of the light emitting device of the invention as examples.

FIGS. 13A and 13B show cross sectional structures of a pixel of the light emitting device of the invention as examples.

FIG. 14 shows a cross sectional structure of a pixel of the light emitting device of the invention as an example.

FIG. 15 shows a cross sectional structure of a pixel of the light emitting device of the invention as an example.

FIG. 16 shows a structure of an external circuit and a panel.

FIG. 17 shows one mode of a signal line driver circuit.

FIGS. 18A and 18B are top plan view and a cross sectional view of the light emitting device of the invention.

FIGS. 19A to 19E illustrate electronic devices using the light emitting device of the invention.

Detailed description of the invention

[Embodiment Mode1]

FIG. 1 shows one mode of a pixel in the light emitting device of the invention. The pixel shown in FIG. 1 includes a light emitting element 101, a transistor (switching transistor) 102 for controlling an input of a video signal to the pixel, a driving transistor 103 for controlling a current value to be supplied to a light emitting element 101, and a current controlling transistor 104 which operates to supply or not to supply current to the light emitting element 101. A capacitor 105 may be provided in the pixel for maintaining the potential of the video signal as in this embodiment mode.

In FIG. 1, the driving transistor 103 and the current controlling transistor 104 may have the same polarity or different polarity. The driving transistor 103 operates in a saturation region as an example in this embodiment mode, however, it may operate in a linear region as well. The switching transistor 102 and the current controlling transistor 104 operate in a linear region. The driving transistor 103 may be an enhancement mode transistor or a depletion mode transistor. The switching transistor 102 may be either an n-type or p-type transistor.

The gate of the switching transistor 102 is connected to a scan line Gj (j=1 to y). One of the source and drain of the switching transistor 102 is connected to a signal line Si (i=1 to x) and the other is connected to the gate of the current controlling transistor 104. The gate of the driving transistor 103 is connected to a second power supply line Wi (i=1 to x). The driving transistor 103 and the current controlling transistor 104 are connected to a first power supply line Vi (i=1 to x) and the light emitting element 101 so that the current supplied from the first power supply line Vi (i=1 to x) is supplied to the light emitting element 101 as a drain current of the driving transistor 103 and the current controlling transistor 104. In this embodiment mode, the source of the current controlling transistor 104 is connected to the first power supply line Vi (i=1 to x) and the drain of the driving transistor 103 is connected to a pixel electrode of the light emitting element 101.

It should be noted that the source of the driving transistor 103 may be connected to the first power supply line Vi (i=1 to x) and the drain of the current controlling transistor 104 may be connected to the pixel electrode of the light emitting element 101.

The light emitting element 101 includes an anode, a cathode, and an electroluminescent layer formed between the anode and the cathode. One of the anode and the cathode is the pixel electrode and the other is a counter electrode.

One of two electrodes of the capacitor 105 is connected to the first power supply line Vi (i=1 to x) and the other is connected to the gate of the current controlling transistor 104. The capacitor 105 is provided in order to hold the gate voltage of the current controlling transistor 104. Note that the capacitor 105 is provided in FIG. 1, however, the invention is not exclusively limited to this configuration and the capacitor 105 may not necessarily be provided.

In the case of using p-type transistors as the driving transistor 103 and the current controlling transistor 104 as in FIG. 1, the drain of the driving transistor 103 and the anode of the light emitting element 101 are desirably connected to each other. That is to say, it is desirable to use the anode as the pixel electrode and the cathode as the counter electrode. In the case of using n-type transistors as the driving transistor 103 and the current controlling transistor 104, on the other hand, the source of the driving transistor 103 and the cathode of the light emitting element 101 are desirably connected to each other. That is to say, it is desirable to use the cathode as the pixel electrode and the anode as the counter electrode.

A driving method of the pixel shown in FIG. 1 is described now. The operation of the pixel shown in FIG. 1 can be described in two periods: a write period and a store period. FIG. 2A shows the operation when the current controlling transistor 104 is ON in the write period, and FIG. 2B shows the operation when the current controlling transistor 104 is OFF in the write period. FIG. 2C shows the operation when the current controlling transistor 104 is ON in the store period and FIG. 2D shows the operation when the current controlling transistor 104 is OFF in the store period. Note that the switching transistor 102 and the current controlling transistor 104 are shown simply as switches in FIGS. 2A to 2D in order to simplify the operations.

In the write period, current supply to the light emitting element 101 is stopped regardless of the switching of the current controlling transistor 104. Specifically, potential difference between the counter electrode of the light emitting element 101 and the first power supply line Vi (i=1 to .times.) may be zero. Otherwise, potential difference between the counter electrode and the first power supply line Vi (i=1 to .times.) may be controlled so that a reverse bias voltage is applied between a pair of electrodes of the light emitting element 101 when it is considered as a diode. Alternatively, current path to the light emitting element 101 may be blocked by a switch and the like. When a scan line Gj (j=1 to y) is selected, the switching transistor 102 of which gate is connected to the scan line Gj (j=1 to y) is turned ON. Then, a video signal inputted to the signal line Si (i=1 to .times.) is inputted to the gate of the current controlling transistor 104 through the switching transistor 102. The second power supply line Wi (i =1 to .times.) constantly applies a potential to the gate of the driving transistor 103 which is high enough to turn ON the driving transistor 103 when the current controlling transistor 104 is ON.

It should be noted that the current supply to the light emitting element 101 is stopped in the write period when the current controlling transistor 104 is ON as shown in FIG. 2A and when it is OFF as shown in FIG. 2B in accordance with the potential of a video signal. Therefore, all the light emitting elements 101 are in non-light emitting state in the write period. The potential video signal written in the write period is held by controlling the potential of the scan line Gj (j=1 to y) to turn OFF the switching transistor 102.

In the store period, potential difference which is high enough to supply a forward bias current to the light emitting element 101 is provided between the counter electrode of the light emitting element 101 and the first power supply line Vi (i =1 to x), therefore, current flows to the light emitting element 101 when the current controlling transistor 104 is ON.

Therefore, in the case where the current controlling transistor 104 is ON, current is supplied to the light emitting element 101 through the first power supply line Vi (i=1 to x) as shown in FIG. 2C. The current supplied to the light emitting element 101 is determined by the drain current of the driving transistor 103 and the V-I characteristics of the light emitting element 101. The light emitting element 101 emits light at a luminance according to the supplied current. On the other hand, in the case where the current controlling transistor 104 is turned OF in the write period as shown in FIG. 2D, current supply to the light emitting element 101 is stopped because the potential of the video signal is held in the capacitor 105. Therefore, the light emitting element 101 remains the non-light emitting state.

FIG. 3A shows a configuration of a switch as an example in the case of stopping the current supply to the light emitting element 101 by making no potential difference between the counter electrode of the light emitting element 101 and the first power supply line Vi (i=1 to x) in the pixel shown in FIG. 1. By changing over a switch 110 as shown in FIG. 3A, a potential Vdd is applied to the first power supply line Vi (i=1 to x) and the counter electrode of the light emitting element 101 in the write period, and a potential Vss is applied to the counter electrode of the light emitting element 101 and a potential Vdd is applied to the first power supply line Vi (i=1 to x) in the store period so that a forward bias current can be supplied to the light emitting element 101.

FIG. 3B shows a configuration of a switch in the case of stopping the current supply to the light emitting element 101 by blocking the current path to the light emitting element 101 in the pixel shown in FIG. 1. As shown in FIG. 3B, the counter electrode floats by blocking the current path to the light emitting element 101 by turning OFF the switch 111 in the write period, and current flows to the light emitting element 101 by turning ON the switch 111 in the store period so that a forward bias current can be supplied to the light emitting element 101.

Timing of the write period and the store period as an example is described with reference to FIG. 4.

FIG. 4 shows an example of displaying a 4-bit gray scale by using time gray scale method. Ts1 to Ts4 are store periods corresponding to each bit. The length of the store periods are in the following ratio. Ts1:Ts2:Ts3:Ts4=2.sup.3:2.sup.2:2.sup.1:2.sup.0=8:4:2:1. Tb1 to Tb4 correspond to write periods per row of pixels aligned along the scan line corresponding to each bit. Ta1 to Ta4 correspond to the total write periods from the start of writing corresponding to each bit until finishing the writing of all lines of pixels.

In the write period Tb1, scan lines are selected from the pixels of the first row to turn ON the switching transistors. Subsequently, video signals are inputted to each pixel from the signal line. Once video signals are inputted, the write period Tb1 terminates in that row and the potential of the video signal is held. This operation is performed up to the last row and the period Ta1 terminates. Subsequently, the store period Ts1 starts in all rows. In the store period, light emission and non-light emission of each pixel are controlled by the potential of the video signal inputted in the write period Ta1. After the store periods terminate in all pixels all at once, the write period Tb2 corresponding to the next bit starts from the pixels of the first row again.

Description was made on the case of displaying a 4-bit gray scale, however, the number of bits and gray scale are not limited to this. Further, the order of the store periods does not have to be Ts1 to Ts4. It may be randomly ordered or each store period may be divided into a plurality of periods for performing display.

When the current controlling transistor 104 operates in a linear region, the drain voltage Vds thereof becomes extremely small relatively to a voltage Vel applied to the light emitting element 101. Thus, a slight change in a gate voltage Vgs does not easily affect the current supplied to the light emitting element 101. The gate potential of the driving transistor 103 is fixed without being controlled by video signal. Therefore, the current supplied to the light emitting element 101 does not change easily even without increasing the capacitance of the capacitor 105 provided between the gate and source of the current controlling transistor 104, or suppressing the off-current of the switching transistor 102 low. The current supplied to the light emitting element 101 is not affected by the parasitic capacitance of the gate of the current controlling transistor 104. The current controlling transistor 104 only operates to supply or not to supply a current to the light emitting element 101. The current value to be supplied to the light emitting element 101 is determined by the driving transistor 103. Therefore, causes of the variation are decreased and the image quality can considerably be enhanced. Moreover, the process does not have to be optimized for suppressing the off-current of the switching transistor 102, therefore, fabrication process of the transistors can be simplified, which contributes to the reduction in cost and improvement in yield.

The driving transistor 103 operates desirably in a saturation region, however, it may operate in a linear region as well. The drain current is apt to be affected by the slight change in the gate voltage Vgs in a saturation region more easily than in a linear region. However, the gate potential of the driving transistor 103 is fixed in the invention, therefore, the gate voltage Vgs does not change easily even when the driving transistor 103 operates in a saturation region. When the driving transistor 103 operates in a saturation region, the drain current is not changed by the drain voltage Vds, but determined only by Vgs. Therefore, value of the drain current is maintained relatively constant even when Vds gets small instead of Vel getting large in accordance with the degradation of the light emitting element. Therefore, the reduction in luminance of the light emitting element and the appearance of the luminance variation due to the degradation of the electroluminescent material can be suppressed.

In an active matrix light emitting device, current supply to the light emitting element can be maintained to some extent even after the video signal is inputted. Therefore, it can be flexibly applied to a large panel and high precision and it is becoming a mainstream in the future. A specific pixel configuration of the active matrix light emitting device varies according to manufacturers and each manufacturer exercises its ingenuity. FIG. 5 shows a systematic classification of the driving methods of the active matrix light emitting device.

As shown in FIG. 5, driving method of an active matrix light emitting device 501 is roughly classified into two: the one with digital video signals 502 and the one with analog digital signals 503. Furthermore, an analog light emitting device is classified into a current modulation type 504 in which the current value supplied to a light emitting element is modulated in an analog manner, and a time modulation type 505 in which the gray scale is displayed by changing the ON/OFF periods of an inverter. A light emitting device of the current modulation type can also be classified into the one having a Tr characteristic correction circuit 505, and the one having no Tr characteristic correction circuit 506. The Tr characteristic correction circuit is a circuit for correcting characteristic variation of driving transistors, such as a circuit for correcting only the threshold voltage or a circuit for correcting the current value (including the threshold voltage, the mobility, and all the other like).

The light emitting device having the Tr characteristic correction circuit which is classified as the current modulation type is further classified into the one in which the threshold voltage is corrected by a voltage programming 508 and the one in which the current value is corrected by a current programming 509. In the voltage programming, video signals are inputted with voltage, thereby correcting variation in the threshold voltage of a driving transistor. On the other hand, in the current programming, video signals are inputted with current, thereby correcting variation in the current value (including the threshold voltage, the mobility, and all the other like) of a driving transistor. Since a light emitting element is a current driving element and its luminance intensity is determined by a current value, current value may be directly used as data.

The light emitting device in which the current value is corrected by a current programming is further classified into a current mirror type 510 and non-current mirror type 511. In the light emitting device of the current mirror type, a transistor for setting current and a transistor for supplying current to a light emitting element are separately disposed in a pixel circuit using a current mirror circuit. It is an initial premise that the two transistors have the identical characteristics. In the light emitting device of the non-current mirror type, a current mirror circuit is not used and current setting and current supply to a light emitting element are controlled by using one transistor.

On the other hand, a digital light emitting device is classified into the one using an area gray scale method 512 and the one using a time gray scale method 513. According to the area gray scale method, each pixel includes sub-pixels whose light emission areas are sectioned by the square as 1:2:4:8: and . . . , then the gray scale is displayed by selecting them. The area gray scale method includes a gate potential fixation method during light emission 514. According to the gate potential fixation method during light emission, Vgs of the driving transistor is maintained constant by fixing the gate potential of the driving transistor during light emission period of the light emitting element to improve the display defect. The video signals are inputted to the gate of the current controlling transistor connected in series to the driving transistor.

According to the time gray scale method, one frame includes several sub-frames whose light emission period is sectioned by the square as 1:2:4:8: and . . . , then the gray scale is displayed by selecting them. The time gray scale method is also classified into a DPS (Display Period Separated) drive 515 and an SES (Simultaneous Erasing Scan) drive 516. According to the DPS drive, each sub-frame includes two periods: a data write period (Addressing Period) and a light emission period (Lighting Period). The DPS drive is disclosed in "M. Mizukami, et al., 6-Bit Digital VGA OLED, SID00 Digest, p.912". The DPS drive includes the gate potential fixation method during light emission which is described above 517. The invention is classified into the gate potential fixation method during light emission of DPS drive.

According to the SES drive, the data write period and the light emission period can be overlapped with each other by using an erasing transistor, thus a light emitting element can emit light for a longer period. The SES drive is disclosed in "K. Inukai, et al., 4.0-in. TFT-OLED Displays and a Novel Digital Driving Method, SID00 Digest, p.924". The SES drive is further classified into a constant current drive and a constant voltage drive. According to the constant current drive, a light emitting element is driven at a constant current, in which a current can be supplied constantly without being affected by the resistance change of a light emitting element. According to the constant voltage drive, a light emitting element is driven at a constant voltage. The constant voltage drive includes the gate potential fixation method during light emission which is described above 520.

The constant current drive light emitting device is classified into the one having a Tr characteristic correction circuit 521, and the one having no Tr characteristic correction circuit 522. As a light emitting device having the Tr characteristic correction circuit here is a light emitting device of a drive (CCT1) as disclosed in International publication WO 03/027997 and a light emitting device of a drive (CCSP) as disclosed in Japanese Patent Laid-Open No.2003-255896. The light emitting device having no Tr characteristic correction circuit is further classified into the one comprising a driving transistor with a long channel length and the one using a gate potential fixation method during light emission. The light emitting device using the gate potential fixation method during light emission may have a long channel length. The display device comprising a driving transistor with a long channel length is disclosed in Japanese Patent Laid-Open No. 2003-295793. According to the display device comprising a driving transistor with a long channel length, characteristic variation of driving transistors driven at a constant current drive are suppressed. When the gate length is designed extremely long, Vgs in the vicinity of the threshold voltage is not used, thus it becomes possible to reduce variation in current value supplied to the light emitting element in each pixel.

FIG. 6 shows a list of the driving methods classified by voltage or current of the video signals in the light emitting device with digital video signals. As shown in FIG. 6, there are the one in which video signals are inputted with constant voltage (CV) to the pixels and the one in which video signals are inputted with constant current (CC) when the light emitting element emits light.

The driving method in which video signals are inputted with constant voltage (CV) includes the one in which constant voltage is applied to the light emitting element (CVCV) and the one in which constant current is supplied to the light emitting element (CVCC). Further, the driving method in which video signals are inputted with constant current (CC) includes the one in which constant voltage is applied to the light emitting element (CCCV) and the one in which constant current is supplied to the light emitting element (CCCC).

The light emitting device of the invention is classified into CVCV when the driving transistor operates in a linear region, and CVCC when it operates in a saturation region.

[Embodiment Mode2]

In this embodiment mode, one mode of a pixel in the light emitting device of the invention which is different from Embodiment Mode 1 is described.

FIG. 7A shows a pixel configuration according to this embodiment mode. The pixel shown in FIG. 7A includes a light emitting element 201, a switching transistor 202 used as a switching element for controlling an input of a video signal to the pixel, a driving transistor 203 for controlling a current value to be supplied to the light emitting element 201, and a current controlling transistor 204 for controlling the current supply to the light emitting element 201. A capacitor 205 for holding a potential of the video signal may be provided in the pixel as in this embodiment mode.

The driving transistor 203 and the current controlling transistor 204 may have the same polarity or different polarity. The driving transistor 203 may operate in either a linear region or saturation region. The switching transistor 202 and the current controlling transistor 204 operate in a linear region. The driving transistor 203 may be an enhancement mode transistor or a depletion mode transistor. The switching transistor 202 may be either an n-type or p-type transistor.

The gate of the switching transistor 202 is connected to a first scan line Gaj (j=1 to y). One of the source and drain of the switching transistor 202 is connected to the signal line Si (i=1 to x) and the other is connected to the gate of the current controlling transistor 204. The gate of the driving transistor 203 is connected to a second scan line Gbj (j=1 to y). The driving transistor 203 and the current controlling transistor 204 are connected to a power supply line Vi (i=1 to x) and a light emitting element 201 so that the current supplied from the power supply line Vi (i=1 to x) is supplied to the light emitting element 201 as a drain current of the driving transistor 203 and the current controlling transistor 204. In this embodiment mode, the source of the current controlling transistor 204 is connected to the power supply line Vi (i=1 to x) and the drain of the driving transistor 203 is connected to a pixel electrode of the light emitting element 201.

It should be noted that the source of the driving transistor 203 may be connected to the power supply line Vi (i=1 to x) and the drain of the current controlling transistor 204 may be connected to the pixel electrode of the light emitting element 201.

The light emitting element 201 includes an anode, a cathode, and a electroluminescent layer formed between the anode and the cathode. One of the anode and the cathode is the pixel electrode and the other is a counter electrode.

One of two electrodes of the capacitor 205 is connected to the power supply line Vi (i=1 to x) and the other is connected to the gate of the current controlling transistor 204. The capacitor 205 is provided in order to hold the gate voltage of the current controlling transistor 204. Note that the capacitor 205 is provided in FIG. 7A, however, the invention is not exclusively limited to this configuration and the capacitor 205 may not necessarily be provided.

In the case of using p-type transistors as the driving transistor 203 and the current controlling transistor 204 as in FIG. 7A, the drain of the driving transistor 203 and the anode of the light emitting element 201 are desirably connected to each other. That is to say, it is desirable to use the anode as the pixel electrode and the cathode as the counter electrode. In the case of using n-type transistors as the driving transistor 203 and the current controlling transistor 204, on the other hand, the source of the driving transistor 203 and the cathode of the light emitting element 201 are desirably connected to each other. That is to say, it is desirable to use the cathode as the pixel electrode and the anode as the counter electrode.

A driving method of the pixel shown in FIG. 7A is described now. The operation of the pixel shown in FIG. 7A can be described in two periods: a write period and a store period as was in the case of Embodiment Mode 1.

The description continues in the full USPTO document.